"Oppie" wrote:

"ignator" wrote in message
...

Someone here will probably beat me up over this.
ignator

Found this link
http://www1.eere.energy.gov/manufact...ployment/pdfs/
motor_tip_sheet14.pdf
This references standing waves with long cables. Maybe that's why
I've been lucky. I'm just using standard SO cord to connect the mains
to the VFD and VFD to the motor.
ignator


Informative, Thank you!

I had read in the past that putting a "non-inverter" qualified motor on a
VFD caused the motor to overheat (due to iron hysteresis and eddy current
losses) from the high frequency harmonics
really only an issue if you are trying to squeeze every little bit of
power out of the motor. AS I understand them,
"inverter duty " motors largely have better insulation.

and bearings to fail from
circulating currents that caused current flow through the bearings.
This can happen to motors run from any source, but is worse in motors
run from a vfd, but not really a big deal IMO, unless again, you are
running the motor to the ragged edge.
The
exception was if a "line reactor" (basically a 3 ph inductor) was added
between the VFD and motor to smooth out the high frequency content of the
drive waveform.
That is not really why they are there.
The motor itself acts as sufficient inductance to smooth out the
current. It is the voltage that is a problem that the line reactors
(which are just 3 inductors) are meant to solve.

The sharp rise time of the voltage wave forms are more stressful to
the insulation. The frequency is also an issue (switching frequency
that is) because with LONG cable lengths you can reach a point where
the Cable capacitance, and the system inductance resonate, causing
MUCH worse voltage stress at the motor. Unless you are putting the
motor 100s of feet away from the VFD, I wouldn't worry. It is an
issue for something like a down hole pump.
You seem to have disproved this though. VFD technology is
rapidly changing and hard to keep up with.

He doesn't really disprove it, just shows how much of a non issue it
really is for his sort of usage.

If he wants to run his motors at max power or 24/7 then the issue
could be more critical.

Simple basic motors, in basic applications are run from VFDs all the
time in the real world, without a problem.

I had originally posted this inquiry to news:sci.electronics.design under
thread "Soft starter or VFD" and added a bunch of links for soft starters I
had found.

Cheers - Oppie
As A PS, Soft starters are not intended to be SOFT for the Load, but
instead are SOFT (ish) for the power system that feeds them. They may
not do a whole lot for your issue.
jk